I think I've tracked down the ultimate tool for making paper models using Blender. Up until now I've been using UV-unwrap, part of Blender but it turns out that this isn't as accurate as I was hoping. To be fair, it is not intended for the use to which I was putting it. After a bit of searching I can across this Blender Add-On which you can download for free here.

Then it is simply a case of making your model (more of which later) and clicking Save -> Paper Model...